It's not explicitly stated as torture but I also got the vibes from Dark Rendezvous that Dooku didn't exactly have an easy apprenticeship. It makes sense, since both Maul and Palpatine himself were routinely abused both mentally and physically.

I mean in TCW he always came off as lazy and unconcerned more than him failing. In the early CW and AotC he and sheev have a more "friendly" relationship with the latter calling him "my friend" on at least one occasion. In the later CW their relationship appeared to be strained by the time of late season 3 when the whole Ventress threat came up.

Dooku seemed to have grown a little cocky by this time and maybe he needed that choking by Sheev to remind him he's his b1tch. Then he kicked ass on oba diah as asked.

__________________"Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."